The detailed mechanisms by which respiratory and photosynthetic electron transfer chains act as proton pumps are poorly understood. We have recently shown that the ubiquinol:cyt c2 oxidoreductase of Rps. sphaeroides is essentially similar to the mitochondrial ubiquinol:cyt c oxidoreductase, and proposed a modified Q-cycle mechanism which accounts well for the functional characteristics of both complexes. The model provides a preliminary physico-chemical description of the electron transport chain and its operation as a proton pump. The quinone pool, acting as a shuttle of redox equivalents between catalytic sites on the reaction center and the complex, is of central importance in the model. We will study the second order processes by which the pool reacts at these sites, the role of semiquinone stabilization in the catalytic mechanism, and the sites and mechanism of action of inhibitors. We anticipate that detailed thermodynamic and kinetic studies, and computer modelling will provide for the first time a comprehensive physico-chemical description of an electron transfer chain. An understanding of cellular processes at a molecular level is a fundamental prerequisite of rational medicine. Electron transport and energy conversion are major metabolic processes which function by similar mechanisms in all living cells. The advantages of using the photosynthetic bacteria arise from the experimental convenience of initiating electron transport by photochemical activation, and the use of a bacterial source to explore an electron transport chain which is functionally the same as the mammalian mitochondrial complex III.